As an integral part of our study on the functional organization of the nervous control of the circulation, we will investigate the modulatory influence of the following three neural systems on the carotid sinus baroreceptor reflex: the trigeminal depressor, fastigial pressor and cerebral ischemic response systems. The three systems, all of which have been found and/or analyzed by our own group, are powerful neural control mechanisms of the circulation. Our research plan consists of three steps. First, we will advance our ongoing study on hemodynamic and neurophysiological characterization of the trigeminal depressor system. Secondly, we will obtain a comprehensive characterization of the neural modulation of baroreceptor reflex performance. For this purpose we will determine input-output relationships of the baro-receptor reflex with and without neural modulatory influences; we will isolate and force the carotid sinus with the servo-controlled pressure (input) while measuring various hemodynamic responses (output; e.g., arterial pressure, heart rate, total peripheral resistance and regional blood flows and resistances). Neural modulation will be defined as the alterations in input-output relationships produced by manipulations of the neural system. The third step will be to determine the neural site of interaction between the structures in question and the baroreceptor reflex, and further to determine the neural connections between the structures and the neural site of interaction by techniques of focal electrical stimulation, discrete neural lesions, recording of action potentials and detailed histological analysis.